1. Field of the Invention
The present invention relates to a system and method for improved recovery from a tracking error when writing data to a storage medium.
2. Background
A number of strategies for storing data on various types of storage media have been developed. Whether using sequential or random access to retrieve data, most strategies attempt to store as much data in contiguous sections or blocks of the media to better utilize the media capacity, decrease seek time, and increase throughput, i.e. to improve overall performance of the storage and retrieval process. In addition, most strategies use some form of error detection and/or correction and error recovery mechanisms to insure the integrity of data during storage and subsequent retrieval. For example, a read back check may be used to read stored data immediately after writing the data to detect and/or correct corrupted data, which is then rewritten on the fly, i.e. without stopping the recording medium.
As data density increases, proper read/write head positioning becomes more critical to reliably storing and retrieving data. This is particularly true for systems which read and/or write multiple tracks or channels of data simultaneously because positioning errors affect data on all of the multiple tracks or channels. Storage devices have been developed which use one or more read/write head tracking channels to provide proper head positioning during reading and writing of data. These tracking channels do not store user data, but provide feedback for the position of the read/write head relative to the storage medium so that fine positioning adjustments of the read/write head can be made.
Storage devices incorporating track following technology must be able to quickly detect when the recording head is moving off-track to prevent overwriting of existing data. An off-track event may be indicated due to a media defect or anomaly in the servo track that prevents sufficient information from being available to validate proper head positioning. In addition, the positioning system itself may indicate an off-track event or tracking error by incorrectly sensing that the read/write head is about to go off-track. Regardless of the cause of an actual off-track event or false indication of an off-track event, the system must respond by immediately suspending the writing of data to avoid over-writing adjacent tracks or channels that may contain customer data. Once the writing has been interrupted by the off-track indication, the device is stopped, repositioned and the entire data block is rewritten. This recovery procedure is usually successful provided the event was not caused by media defects.
Each time an off-track event actually occurs, or is inaccurately detected, the error recovery process of stopping the media, repositioning, and re-recording the entire data block reduces the performance of the system. As track density continues to increase, the corresponding head positioning tolerances are likely to trigger more actual or perceived off-track events that may significantly impact performance of various types of recording media, particularly tape media.